Mixed carbonate-siliciclastic successions result from the interplay between carbonate production and siliciclastic input, and can be developed by coeval sedimentation (mixed systems), or by an alternation of siliciclastic- and... [ view full abstract ]
Mixed carbonate-siliciclastic successions result from the interplay between carbonate production and siliciclastic input, and can be developed by coeval sedimentation (mixed systems), or by an alternation of siliciclastic- and carbonate-dominated systems (small-scale sequences). This contribution presents preliminary results of a detailed facies and sequence-stratigraphic analysis of a 3-km-long transect in the Upper Member of the Mulichinco Formation (Valanginian, Neuquén Basin), which allows to identify both reciprocal and coeval carbonate-siliciclastic sedimentation.
The investigated succession is composed of ten small-scale cycles (2-20m), comprising a basal, carbonate-rich, transgressive hemicycle, and an upper, siliciclastic-rich regressive hemicycle. The transgressive nature of the carbonate packages is suggested by relationships with underlying and overlying siliciclastic facies, and their basal bounding surfaces.
In the first seven cycles, carbonate hemicycles are characterized by epibenthic autochthonous fossil associations, where two types of shells concentrations were recorded. One type mostly comprises oysters and serpulids floating in a micrite matrix, whereas the other is dominated by coral accumulations surrounded by a mixture of micrite, ooids and subordinated siliciclastics with ripple cross-lamination. These concentrations are thought to represent distal (type 1) and intermediate (type 2) conditions of a carbonate low-energy system, developed during transgressive periods. The siliciclastic hemicycles show a shallowing-upward trend from basal mudstones, passing into sand-rich heterolithics with ripple cross-lamination, and finally to amalgamated HCS-sandstones. They record the progradation of an offshore to lower-shoreface siliciclastic system with no evidence of coeval carbonate production.
The three uppermost cycles are different in several aspects. Carbonate intervals are characterized by amalgamated trough and sigmoidal cross-stratified beds, composed of well-sorted, sand-size ooids, bioclasts and siliciclastics. Occasionally, intercalation of finer grained deposits with abundant micrite occur. The regressive hemicycles comprise coarsening-upward packages of heterolithics, amalgamated HCS-sandstones, and fine-grained sandstones with SCS and trough cross-stratification, but a moderate contribution of ooids (± bioclasts) is always present. Regressive hemicycles reflect progradation of a similar siliciclastic-rich system, but with coeval, albeit subordinated, carbonate production. On the other hand, overlying carbonate intervals reflect the development of complex subaqueous dunes under unidirectional flows, probably representing tide-dominated oolitic shoals away from the shoreline. This, combine with the abrupt decline in siliciclastic supply, points to the transgressive origin.
This study highlights that the role of carbonate production in mixed successions can be highly variable. In this case, biogenic factories were common in distal-middle parts of low-energy carbonate systems, whereas non-biogenic production were important in storm-dominated mixed systems and in high-energy current-dominated carbonate systems.
Topics: Shelf and shallow water sedimentation , Topics: Cycles and rhythms in sedimentary record
